This is one of those quests that almost made me delete the game. So many things that made it so difficult. The goals of the quest are not that difficult in themselves, it's just the problems that arise when things go wrong. So summing up a few tips, these may save you some time.
I died when I first did it, not understanding the mechanics. I COULD NOT find my way back to my body, damn Blizz and the levels. (probably should have paid more attention to where we were going but, hey, I was following her) It all looks so different in the spirit world as well. So I ended up taking res sickness and hs back to Dal. Now you won't get a FP back but you can go to Garden of the Moon fp and just run straight back, however finding the TINY cave entrance is another problem. I ended up abandoning the quest so I could see the ! and still struggled. The flightpath won't appear until you finish the chain tho it shows on the map so that's no landmark. The location coords don't help much either so VISUALLY, keep taking the straight path from Garden of the Moon to Suramar. When you go under the stone arch stop at about 38,48 and look right. There is a natural gap in the rocks, go thru it. You will come to a bridge, go right in the water and run along towards a double waterfall. Right before the waterfalls look right at 34,48 you will see two pillars and the entrance is between them.
Secondly you may run into a problem when you kill Selentia with there not being enough crystals in the middle. There aren't any more in the place, don't waste time like I did running around looking, so as some else said, go back to Thalyssra abandon the quest, exit game, and return. I had to do this twice, getting a variation in number available.
Third, concentrate on killing the tethers first, then she goes down pretty easily. Having done this on a few now, works well to just stand in the hallway. Send your pet, minion, bodyguard after the tethers. Ranged keep doting them from the hallway, and melee just give them a smack when they pass by. None of the banshees will bother you if you stay out in the passage. You can collect the crystals after she dies, but I think you have to try to take one to aggro her.
Good luck